Spatial light modulators (SLMs) including digital micromirror devices (DMDs), liquid crystal display (LCD) devices and liquid-crystal-on-silicon (LCOS) devices are useful in displaying images using optical projection. The optical projection systems (which can include televisions, video displays, projectors and portable projectors) include two parts, which are: an illumination system for generating and collecting the light rays needed to illuminate an image; and a projection system for collecting the illuminated image light rays into a projection lens that projects the final image for viewing. At least one SLM receives the illumination light rays from the illumination system, modulates the light received with image data for visual display, and transmits modulated image light rays for projection to the projection system. In many current systems, the SLM is an array of reflective pixel elements. In certain examples, the SLM pixel elements can be digital micromirrors of a DMD that receive digital image data and move to either reflect or not reflect the illumination light, forming an image corresponding to the digital image data.
In conventional video projection systems using SLMs, image data for projection are can be arranged in frames. The frames are subdivided into bit frames that are projected in sequences during a frame display time. To provide continuous motion to a human eye, a frame rate of at least 60 frames per second (FPS) must be used. However, using faster frame rates, such as 120 FPS, improves the image appearance by reducing visible artifacts (visible artifacts are unintended visible elements that can appear in the displayed image due to the physical characteristics of the display system). Each bit plane can be displayed for a portion of a frame display time. Also, a time weighted display of the bit planes can be used to provide various intensity values. If a bit plane is shown for a greater portion of a frame display time, then a greater intensity will be observed in the displayed image. Pulse width modulation (PWM) of display sequences for the bit frames provides intensity control of the bit planes. Further, sometimes bit planes are split. When split bit planes are used, a sequence for bit plane display can repeat for those split bit planes in a frame that have a higher intensity weighting.
In conventional digital image projection systems, visible artifacts can appear to the viewer in the displayed image at “code boundaries.” Code boundaries occur where the set of bit planes forming adjacent gray shades are very out of phase (time phase) with one another. Examples of these visible artifacts include PWM temporal contour artifacts (or simply “PWM artifacts”). The PWM artifacts are associated with contouring in the displayed image, because an apparent physical boundary can appear if viewers move their eyes around a contour area in the displayed image. A viewer's eyes are simultaneously collecting and integrating light from both sides of the code boundary. Because the integrated light seen on both sides of the code boundary is out of phase in time, a PWM artifact can be seen. This PWM artifact is not part of the desired image and should not appear in the displayed image, but is nonetheless visible to the human viewer due to the operations of the conventional image display system.